Writing professor Ken Macrorie once coined the wonderful term "Engfish" to refer to the dead, artificial prose that writing students cobble together to make their teachers shut up and go away:
College students were once third-graders and occasionally wrote [with childlike disinhibition]. Where did they lose that skill? Why? They spent too many hours in school mastering Engfish and reading cues from teacher and textbook that suggested it is the official language of the school. In it the student cannot express truths that count for him. He learns a language that prevents him from working toward truths, and then he tells lies.
I suspect many of the same things happen in other circles, too. Novice fantasy and SF authors, because they have no voice yet, try instead to pre-emptively please their audience by feeding it some version of what they believe it wants. Or, worse, some version of what they think an "author" gives. They end up sounding like Garth Marenghi. The bigger problem is how many established authors also write like this.
Mediocre writing of most any kind has just enough "there" there to pass casual attention, but no real substance. This will fool someone who reads by sliding their eye down the middle of the page, but not an actual Reader, and even casual readers come away empty without knowing why. Mediocre-to-bad SF&F feels like a paste-up of other work in the same vein, assembled mostly to convince the reader this is a real specimen of the genre, or to assuage the author that it is.
Bad writing is not something that just writers do, but is also supported by a culture of propping up bad writing — of not being willing to call out bad writing when it's pointed to as an example of good writing, or the acme of a given genre. I got disgusted with opening best-selling fantasy novels and seeing Engfish constructions that would have embarrassed a high-school writing teacher: "The trees waved in the wind as if they were living things", or "She stared eastward, her expression horrified, eyes wide and sorrowful. It was the face of a child watching a brutal murder that stole her innocence".
Maybe we could call this "fantfacy" writing? It's bad enough that it's graceless and inexpressive; it also lacks what such stories should most traffic in: imagination. It robs the story of any real sense of wonder. Instead of finding insights that grow out of the milieu or the material, the author gropes for tired and thoughtless metaphors. I just feel let down when we get these books that are supposed to be straining at the leash of wonders, and they're written in, well, Engfish.
Wretched prose like this gets a pass because it's attached to best-selling names, and (as Tibor Fischer once put it) in publishing you go from being not published no matter how good you are, to being published no matter how bad you are. So isn't that all the more reason to push back against this kind of writing? Popularity shouldn't be an excuse for laxity of craft.